The present invention relates to pattern inspection for detection of, for example, a defect and foreign matter by using an image of an object obtained in the manner that the object is irradiated with light or laser and is thereby imaged. More specifically, the invention relates to an apparatus and method for inspecting patterns (or, a “pattern inspection apparatus” and a “pattern inspection method”, hereafter) well suited for exterior inspection of, for example, semiconductor wafers, thin-film transistors (or, “TFTs,” hereafter), and photomasks.
As an existing technology, a method of detecting a defect through comparison between an inspection targets image and a reference image is known, as is disclosed in, for example, in Japanese Unexamined Patent Application Publication No. 05-264467 (or, 1996-264467).
According to this disclosed method, inspection target samples with repetitious patterns regularly arranged are serially imaged by a line sensor, respective imaged images are compared with an image with a time lag corresponding to a repetitious pattern pitch, and a mismatch portion is detected to be a pattern defect. An existing inspection method of this type will be described hereinbelow with reference to an exemplified case of exterior inspection of a semiconductor wafer. As shown in FIGS. 2A and 2B, a large number of chips of same patterns are arranged on an inspection target semiconductor wafer. As shown in FIG. 2B, a respective one of the chips can be broadly categorized into a memory mat portion 601 and a peripheral circuit portion 602. The memory mat portion 601 is a group of small repetitious patterns (cells), and the peripheral circuit portion 602, basically, is a group of random patterns. Generally, a portion such as the memory mat portion 601 has high pattern density, and an image taken thereof by a brightfield illumination optical system is dark. In contrast, the peripheral circuit portion 602 has low pattern density, and an image taken thereof is bright.
In the existing exterior inspection, the peripheral circuit portion 602 performs an inspection by comparing same positions of adjacent chips, such as regions 61 and 62 of FIG. 2A, and detects a portion with a brightness difference greater than a threshold value to be a defect. Such inspection will be alternatively referred to as “chip comparison.” The memory mat portion 601 performs an inspection by comparing images of adjacent cells, and similarly detects a portion with a brightness difference greater than a threshold value to be a defect. Such inspection herebelow will be alternatively referred to as “cell comparison.”
In an inspection target semiconductor wafer, planarization by CMP (chemical mechanical polishing) or the like causes delicate differences in pattern film thickness, thereby causing local brightness differences across images of chips in association therewith. As in the existing method, when a portion where the differential value is greater than or equal to a threshold value TH is determined to be a defect, even such a region having such brightness differences is determined to be defect. However, such a defect should not be detected as an essential defect. Such a detection result is caused in association with misreporting. Conventionally, as a method for avoiding occurrence of such a misreported event, a threshold value for defect detection is increased. However, the method causes reduction in the sensitivity, so that a defect corresponding to a differential value of an equivalent level or lower cannot be detected. Further, in the aligned chips shown in FIG. 2A, a brightness difference in association with the film thickness difference can occur, for example, only in an area between specific chips in the wafer or in an area of a specific patter in the wafer. When the threshold value TH is set in accordance with such the local area, the overall inspection sensitivity is significantly reduced or impaired.
Causes of impairing the sensitivity include variations in pattern edge thickness. FIGS. 10A to 10C are schematic view of a semiconductor pattern used as an inspection target. Gates are provided at about a 200 nm pitch, and there is a linewidth variation of about 20 nm. Numeral 51 in FIG. 8C represents a brightness waveform in the direction of comparisons of target patterns across which small linewidth variations are occurring. In this case, there exist brightness value variations. According to the existing method of comparison inspection, when a brightness value variation such as described above is present across images of adjacent cells or adjacent chips, the variation appears as noise during the inspection.
Alternative cases are that a defect of the above-described type can be detected through combination of factors dependant on, for example, the material, surface roughness, size, depth, and the like and factors, such as illumination conditions, dependant on the detection system.